Mr. Nagorski, who was assigned to Moscow in May 1981, said the official Soviet charges against him were ''all bogus'' and no real reasons were spelled out. A Soviet press department spokesman accused him of impersonating a Soviet correspondent during a visit to the Russian city of Vologda last October. A Newsweek spokesman in New York said that and all the other charges were untrue. It was the first time a US correspondent had been ordered expelled since the mid-1970s.